debbie hersman is chairman of the ntsb. >> we have learned from accidents, we have plowed all of those lessons back in. they have been embraced in many cases, and we're not seeing repeats of those same accidents. >> reporter: but experts say two actions in which human failure was at least partly to blame. underscore that, remains the biggest threat to safety. air france 447 crashed into the atlantic in 2009, killing 228. that same year, regional airline, flight 3407, crashed in buffalo, killing 50. >> colgan air was a watershed event in aviation safety, in particular bringing the regional carriers up to standards with the major carriers. >> reporter: the safety culture is spreading, accident rates are dropping, in russia, africa, latin america. the caribbean and asia. despite the improvement in safety, experts say there is a real danger here. just when you start to think it can't happen again is very often when you're most vulnerable. tom costello, nbc news, washington. >> when we continue on this new year's eve, two men who have learned some hard lessons, now passing them on and makin